1. Field of the Invention
The present invention relates to a monolithic capacitor, and more particularly to a monolithic capacitor that may be advantageously used in a high frequency circuit.
2. Description of the Related Art
A monolithic capacitor for use in a high frequency circuit is described in JP 2-256216 A and shown in FIGS. 9A and 9B. FIG. 9A is a plane view showing a first cross-section of the monolithic capacitor 1 and FIG. 9B is a plane view showing a second cross-section of the monolithic capacitor 1 that is different from the first cross-section.
The monolithic capacitor 1 includes a capacitor body 6 having first and second opposing principle surfaces which are rectangular in shape, and four planar side surfaces 2, 3, 4 and 5 extending therebetween. The capacitor body 6 includes a plurality of dielectric layers 7 each lying in a plane which generally extends parallel to the outer main surfaces. The dielectric layers are made, for example, of a ceramic dielectric. At least one pair of first and second inner electrodes 8 and 9, facing each other and separated by a respective dielectric layer 7, are formed in the capacitor body 6 so as to form a capacitor unit. The inner electrodes are also planar and rectangular in shape. As used herein, the term "capacitor unit" refers to a minimum unit in which a capacitance is formed by the pair of the inner electrodes 8 and 9 separated by a respective dielectric layer.
This monolithic capacitor 1 is constructed to reduce an equivalent series inductance (ESL), so as to be applicable for a use in a high frequency range.
Four first lead electrodes 10, 11, 12 and 13 extend from the first inner electrode 8 to the side surfaces 2 and 4. Particularly, lead electrodes 10 and 11 extend to the side surface 2, and lead electrodes 12 and 13 extend to the side surface 4. First external terminal electrodes 14, 15, 16 and 17 are electrically connected to lead electrodes 10, 11, 12 and 13, respectively.
In a similar manner, four second lead electrodes 18, 19, 20 and 21 extend from second inner electrode 9 to the side surfaces 2 and 4. Particularly, the lead electrodes 18 and 19 extend to the side surface 2, but at locations that are different from the locations where the first lead electrodes 10 and 11 extend, and lead electrodes 20 and 21 extend to the side surface 4, but at locations that are different from the locations where the first lead electrodes 12 and 13 extend. Second external terminal electrodes 22, 23, 24 and 25 are electrically connected to lead electrodes 18, 19, 20 and 21, respectively. As a result, external terminal electrodes 22 and 23 are positioned at locations which are different from the locations of external electrodes 14 and 15 on the side surface 2, and the external terminal electrodes 24 and 25 are positioned at locations which are different from those of the first external electrodes 16 and 17 on the side surface 4. With this structure, the plurality of first external terminal electrodes 14-17 and the plurality of second external terminal electrodes 22-25 are alternately disposed side by side.
In FIG. 9A, typical paths and directions of a currents that flow in monolithic capacitor 1 at a given point of time are illustratively shown with arrows. In the state illustrated, the current is flowing from each of the first external terminal electrodes 14-17 toward each of the second external terminal electrodes 22-25. In case of an alternating current, the current flow will reverse direction with the changing polarity of the alternating current.
As current flows through the inner electrodes, a magnetic flux self-inductance component is induced. The direction of the various components of magnetic flux is determined by the direction of the respective components of current.
Referring to FIG. 9A, since the currents flow in multiple directions with a spread of an approximately 180 degrees at the centers of the inner electrodes 8 and 9 as well as at the respective neighborhoods of the relatively centrally located lead electrodes 11, 13, 18 and 20, the various components of magnetic flux are cancelled out, thereby reducing the ESL.
However, since the various components of current do not easily flow at the locations where the lead electrodes do not exist, that is, at the portion of the inner electrodes 8 located adjacent side surface 3 and the portion of the inner electrode 9 located adjacent the side surface 5, there is minimal canceling of magnetic flux in these areas and the desired reduction in ESL is not achieved.
To overcome this problem, one might increase the number of lead electrodes and associated external terminal electrodes. However it is difficult, if not impossible, to increase the number of the external terminal electrodes in view of the dimensional restriction of the monolithic capacitor, and thus this measure is not always adopted. Also, an increase in the number of external terminal electrodes creates a problem in mounting the monolithic capacitor because it is not always possible to form a large number of lands on a circuit board for connecting these external terminal electrodes to the circuit board.
Another known monolithic capacitor designed for use in a high frequency circuit is described in Japanese Examined Patent Publication No. 4-70764 and shown in FIGS. 10A and 10B. FIG. 10A is a plane view showing a first cross-section of the monolithic capacitor 26 and FIG. 10B is a plane view showing a second cross-section that is different from the first cross-section. The first cross-section is located in a plane where a first inner electrode 33 lies. The second cross-section is located in a plane were a second inner electrode 34 lies.
The monolithic capacitor 26 includes a capacitor body 31 having two rectangular principle surfaces which face one another, and four side-surfaces 27, 28, 29 and 30 extending therebetween. The capacitor body 31 includes a plurality of dielectric layers 32 which lie in respective planes extending parallel to the principle surfaces. At least one pair of first and second inner electrodes 33 and 34, facing each other through a respective dielectric layer 32, are provided to form a capacitor unit.
A first external terminal electrode 36 is provided on a corner 35 of the capacitor body 31 where the two adjacent side surfaces 28 and 29 intersect, and a second external terminal electrode 38 is provided on a corner 37 of the capacitor body 31 where the two adjacent side surfaces 27 and 28 intersect. The corners 35 and 37 are adjacent one another.
The first inner electrode 33 includes a first lead electrode 39 that extends to corner 35, so as to be electrically connected to a first external terminal electrode 36. An angular notch is formed in the upper right hand corner of electrode 33 which is adjacent to the corner 35. The second inner electrode 34 includes a second lead electrode 40 that extends to corner 37, so as to be electrically connected to a second external terminal electrode 38. An angular notch is formed in the lower right hand corner of electrode 34 which is adjacent to corner 37.
In FIG. 10A, typical paths and directions of a current that flows in monolithic capacitor 36 is illustratively shown with an arrow. At the instant shown, the currents flow from the first external terminal electrode 36 toward the second external terminal electrode 38. Since the currents path turns back upon itself (reversing by approximately 180 degrees) the components of magnetic flux induced by current flow in the inner electrodes 33 and 34 are cancelled out, thereby reducing the ESL.
However, in order to generate the current flows described above, the notches have to be provided in the inner electrodes 33 and 34 which reduces the capacitance of the capacitor unit.
This capacitor also creates mounting problems. Since the external terminal electrodes 36 and 38 are formed on one side of the capacitor body 31, the other side is easily lifted up when being mounting to the circuit board.